Prior to the present invention, a common approach to the problem of design for molded plastic paint can lid assemblies has been to provide an interference fit--that is, typically a relatively large diameter ridge on the top of the container is forced through a relatively small diameter on the lid so that a peripheral ledge on the lid will snap into place on the underside of a smaller diameter on the container. The design for such an interference fit will typically have a vertical dimension immediately above the ledge which is too short to prevent the sliding off of the lid if the container is subjected to impact. Impact from dropping the container, particularly at an angle, and particularly coupled with hydraulic action of a packaged liquid, will distort the ledge in microseconds and result in almost immediate release of the lid from the ledge.
Interference fits are shown, for example, in U.S. Pat. Nos. 4,356,930, 3,977,563, 4,349,119, and 4,512,494.
I am not aware of an approach in the prior art similar to mine, e.g. where a tapered slot and complementary tapered lock rings are employed to make the seal become tighter as the lid is moved upwardly as by the force of impact.